Wednesday, March 9, 2011

Really, how hard can it be...?

I mean after all Milton, in his book explains the process of re-assembling the truss parts like this.

   "To insure perfect alignment, drift pins made of sharpened trunnels are used and driven into holes which have been well lubricated with heavy cup grease.
   First, two members are lined up with short 7" pins (note: his 120 foot bridge used 3 - 1/2" by 14" chords 32 feet long!). Next, the first layer of lattice is pinned in place with a drift pin 3 - 1/2" longer than the first, and the second layer of lattice is secured with a still longer pin. This takes place in groups located near end joints.
   When the next chord member, which makes the fifth layer, is laid out, a longer and blunter pin is used which forces the first shorter pin out and replaces it through the five "layers". As the final chord members are laid out, the second short pin is replaced with a trunnel if alignment appears correct. From then on, trunnels are used in well greased holes to punch out and replace other drift pins.
   Before full length trunnels can be driven, a block supported by a jack must be placed near the driving area, to back up the timbers and prevent them from bouncing."

Translation - Lay down and pin together the first two chord layers (layers 1 and 2).  Place web layers 3 ans 4 on top and pin them together where they intersect each other (DO NOT pin them to layers 1 and 2 at this time).  Begin to pin one end of top a bottom chords to web lattice. Continue to move along down the web lattice pulling or pushing chords to line up holes and pin with 4 layer long drifts. Camber will now be in chords. Add layer 5 and pin with drifts. Add layer 6 and drive in trunnels. Use petroleum jelly as a lubricant for drifts.

Hint - Unless you plan to do some digging plan ahead by making sure that the false work is high enough to allow for the drifts to be driven out and removed.

So, how hard can it be?? After having read his book several times I think...  The old saying is true.
I hear it, I forget.
I see it, I remember.
I do it, I understand.

Eureka, Got'er !!!

Puzzler - At 2.35 pounds per board foot each 32' long truss will weigh in at around 2800 pounds. Using only manpower how will the re-assembled truss be raised to the vertical position?

Tech Vocab - Ginpole - Lifting Yoke - Cribbing - Block and Tackle

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